This invention relates generally to well logging apparatus, and more particularly to well logging apparatus that include a gadolinium optical interface.
Modern petroleum drilling operations require large quantities of information relating to geological formations and conditions through which the drill is passing. This collection of information is commonly referred to as logging and can be performed by a number of methods. Oil well logging has been known for many years as a technique for providing information to a driller regarding the particular earth formations being drilled. In conventional wireline logging, a probe or sonde housing information sensors is lowered into a bore hole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the bore hole. The sonde is supported by a conductive wireline, which attaches to the sonde at the upper end. Power is transmitted to the sensors through the conductive wireline. Also, the instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline.
One known method of oil well logging includes a fast neutron source in the logging tool. Neutrons from this source are scattered and absorbed in the well bore environment producing gamma rays. These gamma rays are detected by Nal scintillation crystals in the tool and give information on physical traits of the well bore environment. Light produced from scintillations in Nal is transmitted through an optical interface to a photo-multiplier tube. Despite shielding the surfaces of the Nal scintillator that do not couple to the photo-multiplier tube, neutrons can enter through the optical interface. Thermalized neutrons activate the iodine in the Nal scintillation crystals, which then decays with a half life of 25 minutes. As these decays occur, the Nal scintillator detects the radiation emitted and an elevated background count is created. This background count disturbs and skews the measurements of interest.
One known approach to exclude neutrons from the optical end of the Nal scintillator is to wrap the entire photo-multiplier tube in cadmium. This approach has several disadvantages. Cadmium has only moderate capability at absorbing thermal neutrons. Therefore, the detector must be reduced in length to provide space for the amount of cadmium needed to effectively shield neutrons. Also, cadmium is a known carcinogen and is toxic. The cadmium wrapping is external to the detector thereby limiting the space available for the sensor in the logging tool.